1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a method of fabrication and associated sodium-containing particle electrolyte structure.
2. Description of the Related Art
The rechargeable lithium ion battery (LIB) has triggered the portable electronic devices revolution due to its high power density, long cycling life, and environmental compatibility. The rechargeable LIB consists of a cathode (positive electrode) and an anode (negative electrode), separated by a Li+-ion permeable membrane. A solution or polymer containing lithium-ions is also used in the battery so that Li+-ions can “rock” hack and forth between the positive and negative electrode freely. The positive materials are typically transition-metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and their derivatives. Lithium-ions can move in their interstitial space freely and reversibly. The negative electrode materials can use lithium-metal, alloys, and carbonaceous materials. During discharge, Li+-ions are extracted from the negative electrode and inserted into the positive electrode. In the meantime, electrons pass through an external circuit from the negative electrode to the positive electrode and generate electric power. During a charge, ions and electrons move along the reverse direction and go back to their original places.
Although LIBs have been employed successfully over a broad range of commercial applications, the issue of lithium demand, as it applies to both strain on natural resources and potential fluctuations in price, have motivated the development of low-cost, rechargeable battery technologies as alternatives to LIB. In light of this, sodium-ion batteries (NIBs) have received increased attention due primarily to the fact that sodium has comparable properties to lithium but also boasts the benefits of reduced cost and virtually unlimited supply.
Unfortunately, rechargeable batteries that employ lithium metal (or sodium metal) as the anode are subject to failure mechanisms associated with the formation of “dendrites” on the metal anode surface, which arise as a consequence of non-uniform surface deposition (electroplating) during the charge process. In the case of a lithium metal anode, the evidence of dendrite formation is unambiguous and is characterized by the formation of discrete, rigid surface structures capable of physically penetrating through a separator/membrane (interposed between and therefore isolating anode from cathode) to reach the cathode surface [1]. As a result of this contact between anode and cathode, an electrical “shorting” occurs which can degrade battery performance and/or pose significant safety hazards. With respect to using a sodium metal as an anode, the formation of dendritic structures during charging has been shown to proceed with the liberation of sodium particles that are dispersed in the electrolyte due to the fact that sodium is a “softer” metal than lithium. The subsequent migration of suspended Na particles in the liquid electrolyte to the cathode can lead to electrical “shorting” and depletion of anode material (consumption of Na metal), leading to reduced capacity and chemical reaction(s) with the active/inactive components comprising the cathode.
In light of the technical challenges associated with Na dendrite formation during charging, several strategies have been investigated as alternatives to conventional (liquid) electrolyte systems in NIBs. In some cases, polymeric or polymer gel electrolytes and various composites thereof) have been considered since the polymeric matrices are expected to impede the free migration of Na particles [2]. With respect to the polymeric (gel) electrolytes, high porosity and low crystallinity are desired attributes for the polymer matrix, which are correlated with the ability to both take-up and retain large volumes of liquid electrolyte (containing dissolved Na+ salt). Numerous approaches have been reported for creating porous polymer gel electrolytes including, but not limited to, inclusion of plasticizers during processing, integration of ceramic “fillers” to furnish composite materials, the application of phase inversion (PI) techniques, and the flash-freezing of solvated polymer matrices [3, 4].
Of course, the beneficial impact of high porosity is directly related to the existence of conductive channels through which Na+ ions can flow during battery cycling. Unfortunately, the same conductive channels function as pathways through which Na particles can flow from anode to cathode. Therefore, it is likely that there exists an optimal pore size (and/or pore size distribution) for realizing high ionic conductivity for Na+, while at the same time effectively suppressing Na particle migration from anode to cathode.
In contrast, solid polymeric electrolytes offer the advantage of being “liquid-free” so that Na particles liberated from the surface of the sodium metal anode are not free to flow towards the cathode. Unfortunately, solid polymeric electrolytes conventionally demonstrate low ionic conductivities at room temperature, while significant contact resistance arising between polymer electrolyte/electrode interfaces limits battery performance [5, 6].
Finally, solid-state (ceramic, nonpolymer) electrolytes can offer advantages in terms of high Na conductivity while functioning as a physical barrier against Na particle formation/migration. However, these materials suffer from drawbacks that include high manufacturing costs, physical rigidity that places limitations on the battery architecture (or form), and performance/stability that is sensitive to stoichiometry and/or the presence of contaminants (secondary phases) [7, 8].
FIG. 1 is a diagram depicting the crystal structure of a metal hexacyanometallate (MHCM) (prior art). Transition MHCFs can be categorized into a more general group of MHCMs that have the open framework as shown. MHCMs can be expressed as AXM1YM2Z(CN)N·MH2O, where A can be selected from, but not limited to alkali and alkaline metals, and M1 and M2 are transition metals such as titanium (Ti), vanadium (V), chrome (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), calcium (Ca), magnesium (Mg), etc. M1 and M2 can be the same or a different metal. The ratio (X:N) of M1 and M2 varies, depending on the materials used. In addition, various amounts of water (H2O) can occupy in interstitial or lattice positions of MHCMs.
It would be advantageous if an electrolyte and/or ion-permeable membrane existed that promoted the migration metal ions between an anode and cathode, while discouraging the formation of metal dendrites.    [1]K. J. Harry, D. T. Hallinan, D. Y. Parkinson, A. A. MacDowell, N. P. Balsara, “Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes,” Nature Mater. (2014), DOI: 10.1038/NMAT3793.    [2]W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang and J. Zhang, “Lithium metal anode for rechargeable batteries,” Energy Environ. Sci., 7(2014) 513.    [3]D. Kumar, S. A. Hashmi, “Ionic liquid based sodium ion conducting gel polymer electrolytes”, Solid State Ionics, 181 (2010) 416.    [4]S. Samitsu, R. Zhang, X. Peng, M. R. Krishnan, Y. Fujii, I. Ichinose, “Flash freezing route to mesoporous polymer nanofibre networks”, Nature Comm. 4(2013) 2653.    [5]R. C. Agrawal, G. P. Pandey, “Solid polymer electrolytes: materials designing and all-solid-state battery applications: an overview,” J. Phys. D: Appl. Phys. 41(2008) 223001.    [6]M. Patel, K. G. Chandrappa, A. J. Bhattacharyya, “Increasing ionic conductivity of polymer-sodium salt complex by addition of a non-ionic plastic crystal,” Solid State Ionics 181 (2010) 844.    [7]N. Anantharamulu, K. K. Rao, G. Rambabu, B. V. Kumar, V. Radha, M. Vithal, “A wide-ranging review on Nasicon type materials,” J. Mater. Sci. 46 (2011) 2821.    [8]V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonzalez, T. Rojo, “Na-ion batteries, recent advances and present challenges to become low cost energy storage systems,” Energy Environ. Sci. 5 (2012) 5884.